Conventionally, a centrifugal blower is proposed, in which a leakage of an air from a gap between a shroud and a bell mouth of a centrifugal fan is reduced in order to decrease a separation noise on a negative pressure surface of a blade caused by an interference with a main flow (for example, refer to Patent Document 1: JP2001-115991A). Patent Document 1 discloses a labyrinth seal portion provided in a part of the shroud facing to an air intake side end portion of the bell mouth in a blade negative pressure surface area.